Arizona operates a ‘closed primary’ system requiring pre-party registration from voters ahead of the election. Many voters had queued for hours on the day of the vote only to be told they couldn’t vote in the presidential primary as they did not appear on the list of registered voters.

Thousands of other voters were given provisional ballots for the primary which were not counted as their party registration showed up in the system as “independent” or “no party declared”.

“This is something that I know happens and I know it happened to people in this room,” Regan said.

“It’s not hearsay - it happened to someone in my own office. One of my employees was registered as a particular party, went to go vote, and I don’t want to divulge his personal details, but it happened to him.”